Responses to volcanoes in tree rings and models

November 29th, 2012 by gavin

The third possibility is that some tree-ring reconstructions can’t be easily compared to simple temperature averages from the models. As both the original paper and the comment suggests, there are important effects from memory from previous years in ring widths and, potentially, increases in diffuse light post-eruption promoting growth spurts. This needs to be assessed using more sophisticated forward models for tree ring growth applied to the models’ output – a feature in both the Mann et al, and Anchukaitis et al. approaches. More work is likely needed on this, and using the wider variety of model experiments coming out of CMIP5/PMIP3.

There is clearly potential for these competing hypotheses to get sorted out. Information from newly-digitised old instrumental records in the early 19th Century such as shipping records for the East India Company (Brohan et al, 2012), doesn’t support the largest modelled responses to Tambora (1815), but does suggest a response larger and more defined than that seen in some reconstructions. However, other 19th Century temperature compilations such Berkeley Earth show larger responses to Tambora – though there are spatial sampling issues there as well. There is also the potential for non-tree ring based reconstructions to provide independent confirmation of the magnitude of the response.

So while neither of the latest comments and responses provide a definitive answer to the principal problem, there is certainly lots of scope for extended and (hopefully) productive discussions.

49 comments on this post.

bill:

November 29th, 2012 at 11:12 PM

I’m sure you’re well aware of what this relatively minor technical dispute has been beaten up into by the usual suspects! A clear sign of desperation – not to mention an obsession with discrediting Mike Mann – methinks…

Michael Schnieders:

November 29th, 2012 at 11:31 PM

correct me if I’m missing something.
As a tree grows it puts on mass. Some of that mass gained is drawn directly from the air. Concentrations of various identifiable compounds would be deposited within the ring unique to that year; for example, Carbon 15, or Fluorine. if one were to find trees from the approximate time and match up chemical signatures from before and after the questionable ring “slips”…. well you know where i”m going

Russell:

November 30th, 2012 at 12:33 AM

Can you direct us to a quantitative discussion of the short-term efffects of tephra aerosols on surface reaction mediated polar O3 equilibri , and the magnitude of the resulting shift in the surface solar flux at high latitudes?

Aaron:

November 30th, 2012 at 2:22 AM

Bill

I do not think Anchukaitis et al. are part of the any group that is trying to discredit Dr. Mann( et al.). They do have a right to critique Mann et al. on the method of analyzing those samples based on locations that Mann et al. choose.

[Response:: They certainly do. Its all part of the legitimate, honest give-and-take of science that I discuss in my book “The Hockey Stick and the Climate Wars” (http://bit.ly/sRasaq) and which is to be distinguished from the dishonest attacks by the Watts/McIntyres/Singers/Michaels of the world. Those interested in this give-and-talk will surely want to watch my AGU New Fellows talk next week [Abstract Title: The Past as Prologue: Learning from the Climate Changes in Past Centuries (Invited) Final Paper Number: A32D-02, Presentation Type: Oral Presentation, Presentation Date and Time: December 5, 2012; 10:30 AM to 10:50 AM, Presentation Length: 20 minutes, Session Title: A32D. New Atmospheric Sciences Fellows Presentations II (Video On-Demand), Location: 3002 (Moscone West)] which will be livestreamed. I’ll be showcasing some very interesting new results based on real world data (the chronologies used to build D’Arrigo et al ’06) and, yes, Houston–we do very much appear to have a problem. I will also be addressing the recent claim by Esper et al that multiproxy reconstructions are underestimating a long-term cooling trend. This claim will be demonstrated to have been falsified :) – mike]

Rob Wilson:

November 30th, 2012 at 3:43 AM

w.r.t. “the cooling would be sufficient to saturate the growth response, and that SOME trees might `skip´ a ring for that year leading to a slight slippage in tree-ring dating”.

Some trees, all trees, or 50% of the trees? This is one of the main issues. At some sites, some trees indeed will express a locally absent ring at chest height where samples are taken. These can easily be identified through crossdating with the other trees in a stand that have this ring. Even in the unlikely event that all trees at a site may be missing a ring for a particular year, other sites are almost always sampled downslope or away from the high latitude tree-line at sites (as part of a wider network) that will not be quite as sensitive to temperature variability (i.e. growth will be less limited by temperature). These trees and those from other sites in a network can be used to identify the missing ring at that one site. In my 18 years of tree-ring work, I have NEVER seen a stand-wide missing ring.

Let’s finally focus on 1815/16. Accordingly to Mike’s variant of the VS tree-growth model and how he implemented it, ca. 50% of all trees will be missing the 1816 ring. There is simply NO evidence of such a phenomenon in tree-ring records in the regions affected by the “year without a summer” – NW North America and Europe. There are long instrumental and historical records in these regions which clearly show no “slippage” of dating prior to 1816.

Mike – please take the opportunity to speak to your dendrochronological colleagues and friends next week at AGU.
Rob

[Response: All very interesting points Rob. I hope you and your colleagues will watch my AGU New Fellows talk next week [Abstract Title: The Past as Prologue: Learning from the Climate Changes in Past Centuries (Invited) Final Paper Number: A32D-02, Presentation Type: Oral Presentation, Presentation Date and Time: December 5, 2012; 10:30 AM to 10:50 AM, Presentation Length: 20 minutes, Session Title: A32D. New Atmospheric Sciences Fellows Presentations II (Video On-Demand), Location: 3002 (Moscone West)] which will be livestreamed. I’ll be showcasing some very interesting new results based on real world data (the chronologies used to build D’Arrigo et al ’06) and, yes, Houston–we do very much appear to have a problem. I will also be addressing the recent claim by Esper et al that multiproxy reconstructions are underestimating a long-term cooling trend. This claim will be demonstrated to have been falsified :) – mike]

Tas:

November 30th, 2012 at 4:09 AM

Just a note to point that recently published ice core evidence suggests the large globally significant 1450s eruption usually denoted as Kuwae was in the late 1450s, with ice core sulphate deposition around 1458. Later than the 1452-3 date, which is likely to have been a second smaller eruption with more geographically confined signature.

Jan Esper:

November 30th, 2012 at 5:36 AM

Sure, anything is possible. But it doesn’t necessarily have to be published in Nature.

[Response: As we should all be aware, what gets into Nature or Science (or Nature Geoscience) is a bit of mystery – and we could all list many papers that didn’t ‘have’ to be published there. I doubt however they would be the same! More to the point, everyone can submit stuff to these journals and kudos to them if it gets accepted. – gavin]

I recommend going out into the forest, develop a tree-ring chronology, and learn crossdating. There are hundreds of dendrochronologists that did this work in the past and could show you the basic techniques.

You could then check your ideas with real-world data and try to demonstrate that post-volcanic rings are missing, before concluding “the potential biases identified in our study necessarily impact all existing hemispheric-scale estimates” and “bolster the case for a significant influence of explosive volcanism on climate in past centuries”.

[Response: I don’t have any dog in this particular issue and I wrote this to point out this this is part of a bigger discussion. It seems to me that the process of checking these hypotheses is well underway. I will be happy to see further discussion on the various issues. – gavin]

[Response: With regard to checking with real world data, you might want to watch my AGU New Fellows talk next week [Abstract Title: The Past as Prologue: Learning from the Climate Changes in Past Centuries (Invited) Final Paper Number: A32D-02, Presentation Type: Oral Presentation, Presentation Date and Time: December 5, 2012; 10:30 AM to 10:50 AM, Presentation Length: 20 minutes, Session Title: A32D. New Atmospheric Sciences Fellows Presentations II (Video On-Demand), Location: 3002 (Moscone West)] which will be livestreamed. I’ll be showcasing some very interesting new results based on “real world data” and, yes, Houston–we do very much appear to have a problem. I will also be addressing the recent claim by Esper et al that multiproxy reconstructions are underestimating a long-term cooling trend. This claim will be demonstrated to have been falsified :) – mike]

November 30th, 2012 at 8:37 AM

An “a” should be replaced with “in”?

“but for which many paleo-reconstructions barely show a blip a temperature”

should maybe been spelled:
“but for which many paleo-reconstructions barely show a blip in temperature”

[Response:fixed, thanks. – gavin]

flxible:

November 30th, 2012 at 9:38 AM

Lordy! Dr Mann has another set of concentric rings on his backside?

[Response:: At the risk of sounding like a broken record, the shoe may be on the other foot so to speak. Tune in to my AGU New Fellows talk next week [Abstract Title: The Past as Prologue: Learning from the Climate Changes in Past Centuries (Invited) Final Paper Number: A32D-02, Presentation Type: Oral Presentation, Presentation Date and Time: December 5, 2012; 10:30 AM to 10:50 AM, Presentation Length: 20 minutes, Session Title: A32D. New Atmospheric Sciences Fellows Presentations II (Video On-Demand), Location: 3002 (Moscone West)] which will be livestreamed. I’ll be showcasing some very interesting new results based on real world data (the chronologies used to build D’Arrigo et al ’06) and, yes, Houston–we do very much appear to have a problem. I will also be addressing the recent claim by Esper et al that multiproxy reconstructions are underestimating a long-term cooling trend. This claim will be demonstrated to have been falsified :) – mike]

Dan Riseborough:

November 30th, 2012 at 9:52 AM

I just pulled the “Whenever there is a mismatch….” sentence out and made it into a mini-poster to hang over my desk. (With attribution, of course)

November 30th, 2012 at 10:25 AM

How sure are we of the magnitude and composition of the volcanic eruptions?

[Response: The history of eruptions is put together based on networks of sulphate peaks in ice cores which are relatively well-dated and referenced to the historical record. Turning a sulphate peak into an aerosol optical depth in time and space is tricky and the two groups who’ve attempted this took different approaches: either a scaling to Pinatubo (for which we have have good data), or via a model of the the injection. The differences in results – in timing, magnitude, etc. give you a sense of the uncertainties (See Schmidt et al, 2011). So, to answer you’re question, we are mostly (but not exactly) sure. ;-) – gavin]

Tom Scharf:

November 30th, 2012 at 11:04 AM

Fourth possibility: Trees are poor thermometers.

[Response: Read our original paper. We pretty much rule that out for the following reason: the long-term variations in the tree-ring composite match, eerily well, the simulated NH mean temperature history. The forcings used to drive the model simulation are independent of the tree-ring temperatures, so for such a close match to arise by chance alone is highly improbable. As we emphasized in the first paragraph of our original paper (and again in the comment), we find that the only place where there is in fact an obvious mismatch is w.r.t the large volcanic cooling episodes. In followup work we will show at AGU, that mismatch largely goes away when you account for the predicted dating errors in the actual tree-rings that make up the D’Arrigo et al composite. Oops, have I said too much ;) – mike]

Rob Wilson:

November 30th, 2012 at 11:22 AM

Mike,
I think you’re getting a little worked up over this “eerily” good comparison. If you had used the Esper et al (02) NH record, for example, I am sure the match would have been different – probably worse??? Of course, if you had compared DWJ06 to another model, you might equally have gotten an inferior fit to what you showed in February. You yourself in your 2008/9 reconstructions provided an ensemble of reconstruction possibilities, yet now you seem fixated on one model and one NH record (derived using mainly RW data which clearly only had skill at time-scale > 20 years).
Rob

[Response: Thanks for stopping by Rob. If you read our original paper, which I trust you did, you know of course that we didn’t just rely on a single long GCM simulation (CSM). Instead, we did an extensive parallel set of sensitivity analyses using an EBM w/ different estimates of the forcings, different climate sensitivities, etc. and showed that our key conclusions are quite robust. And of course, we analyzed D’Arrigo et al because it was the only pure tree-ring width-based reconstruction available back through the AD 1258 eruption. I have been involved with another team in looking at a full suit of tree-ring and multiproxy reconstructions in the context of comparisons w/ the CMIP5 Millennium simulation results. I will be reporting on that next week at AGU. I hope you and your colleagues will attend my AGU talks next week, or view my live-streamed New Fellow talk. I think you’ll find the new results I present of interest. – mike]

Roddy Campbell:

November 30th, 2012 at 12:11 PM

re Tom #12 inline response – not sure if relevant, but also mismatch in last few decades versus instrument record?

Rob Wilson:

November 30th, 2012 at 12:13 PM

Mike,
I think you will find Esper et al (02) [or the later Frank et al. 07 version] a more “Pure” RW composite. DWJ06 incorporated some MXD data in the Icefields reconstruction, as I am sure you read.
Looking forward to your talk.
Rob

[Response: Thanks for stopping by again Rob. Fair enough, I stand corrected. There are some others we could have used. We thought D’Arrigo et al ’06 was based on perhaps the most extensive compilation, but perhaps not. We do by the way analyze both Frank et al and D’Arrigo et al in the Schurer/Hegerl/Tett/Mann/Phipps analysis I’ll be talking about at AGU. I think you’ll find the results enlightening. – mike]

MMM:

November 30th, 2012 at 2:06 PM

So, I know that CO2 fertilization has been considered in terms of its effects on 20th century trees used in reconstructions – has there been considerations of changes in diffuse light due to aerosol emissions? That strikes me as possibly adding a geographically and temporally heterogeneous factor on top of temperature signals that might be interesting to look at…

[Response: Some dynamic vegetation models do take the diffuse/direct light ratios into account, but these aren’t in widespread use for long millennial runs. The effect is included in some forward models for tree rings (as mentioned above), but I’m not sure if people have yet taken GCMs that have a good representation of that and run them through. Anyone? – gavin]

T Marvell:

November 30th, 2012 at 3:34 PM

A possible theory – volcanos affect temperatures mainly in the Summer (NH), and trees grow mainly in the Spring. Thus, one might expect limited impact. The spring growth and summer growth are evident in the ring patterns; the summer growth is denser and narrower. Is the summer growth especially narrow in the year or two after volcanos?

SCM:

November 30th, 2012 at 4:49 PM

I see Mike is doing his best to ensure sure his AGU talk is well attended by irate dendrochronologists :-)

Tim Osborn:

November 30th, 2012 at 5:36 PM

Though the evidence seems firmly against the idea of stand-wide missing rings as the explanation, the potential model-data discrepancy during volcanic cooling episodes is certainly an issue worth airing.

My scepticism with this suggestion is whether it is really applicable to the trees in question. I can intuitively see why this could work in some closed-canopy, dense forest, perhaps in the tropics, where an individual foliage patch within the canopy might receive direct sunlight only briefly and benefit from increased diffuse light despite a reduction in total solar radiation.

But my sense is that most of the trees in the D’Arrigo et al. and other NH temperature reconstructions, being sampled from nearer the latitudinal or altitudinal tree lines, will be from open canopy forest. Something like the Polar Urals scene in Stepan Shiyatov’s pictures here:

I don’t know the extent to which the other D’Arrigo et al. sites are like this (Rob would know, having seen some of them in the field I expect) and I don’t know enough about vegetation behaviour to be sure, but my guess would be that open canopy forest wouldn’t care more about diffuse light than total light. And I suspect that vegetation models would mostly be tuned to simulate interior forest behaviour and so might not be that informative about vegetation response to changing diffuse/direct light near the tree line with open and sparse forest cover. Maybe others who know more about this can comment on that?

Tim

[Response: Thanks for your comments Tim. I would take issue with your statement that the evidence is firmly against missing rings. I will present some new work next week at AGU (my live-streamed talk) that provides evidence firmly in favor of there being missing rings. I will be very interested in what the dendro community has to say in response to that evidence. As for the diffuse light effect, we used a crude representation of it, but I don’t think its impact on the trees in question can be so readily dismissed. Alan Robock has also published on this issue as it pertains to tree-ring temperature reconstructions. My co-author Jose Fuentes, a biometeorologist, has also done work in this area. -mike]

Steve Bloom:

November 30th, 2012 at 5:53 PM

Shorter Scharf (#12): “Thermometers are poor thermometers, so there.”

November 30th, 2012 at 6:40 PM

> wouldn’t care more about diffuse light than total light.

I’d guess the opposite. Purely from one natural experiment, we drove up to a mountain site one very cold winter full moon evening and made camp in a layer of fog or low cloud — not very thick top to bottom but extensive side to side and very dense.

And the light between the big trees, inside the tent, inside every bush — was astonishingly bright and entirely shadowless.

Every tiny droplet was being its own full moon to all around it.

Dunno how volcanic dust diffuses sunlight — but if it’s anything like as powerful as fog and moonlight, it’d get a _lot_ more light in under any kind or amount of canopy short of total rock ceiling.

Steve Bloom:

December 1st, 2012 at 7:02 AM

This appears to be the link for the lecture, and it’s Pacific Standard time. Anyone who can’t watch it live can see it after 48 hours. I notice there are a large number of other lectures that will be similarly available (link), which is great (and new for this year IIRC).

December 1st, 2012 at 10:46 AM

#21–I’d second that. When I was a kid and awoke during the night, I could tell at once it was snowing hard by the *brightness* of the diffused illumination from the streetlight on the corner. I’d get up, look out the window, and sure enough… it is rather counterintuitive, but there you are.

Susan Anderson:

December 1st, 2012 at 12:52 PM

fwiw, I’d guess volcanic emissions are a good deal more opaque than water vapor.

Bob Loblaw:

December 1st, 2012 at 1:09 PM

Hank, Kevin:

What you are seeing/discussing is multiple reflections, which is commonly known in radiative transfer stuff. Surface reflects 80% (e.g., fresh snow), but low lying cloud (or fog, or falling snow) reflects 80% of that back down, where the surface reflects 80% back up, etc. An infinite series (but with a finite solution), that greatly enhances the surface irradiance. I have personally measured global solar radiation values that were completely diffuse, but came close to clear-sky values because the diffusion was the result of 10-20m surface layer of blowing snow.

Commonly used by the Inuit as a navigation technique: in a kayak at sea level, on an overcast day, the open water leads cause a dark section in the cloud. You can’t see the leads themselves, but you know where they are because they are “mapped” in the cloud layer. I’ve seen that along the coast in Churchill, Man., with shore leads, and I’ve seen the same effect in agricultural areas with snow cover, where the dark roads show up in the cloud.

Tim Osborn:

December 1st, 2012 at 2:16 PM

Mike — I agree that the diffuse light effect can’t be dismissed. I just thought that some commentators may be picturing these trees as growing in a dense, closed canopy forest, but at least some of these aren’t. Whether that makes a material difference, I’m not sure — I don’t know enough about it.

By the way, the photos by Stepan Shiyatov that I linked to show a rather dramatic extension of the forested area, presumably warming related. Top is 1962, bottom is same view but in 2004. You’re probably familiar with them, but perhaps some readers haven’t seen them before. It’d be interesting to see how much more it’s changed in the last 8 years.

Hank — interesting about the fog effects. The volcanic “fog” is about 10km above the trees. Would that make a difference?

[Response: ok, thanks for the followup & clarification Tim. In a cab on my way to hotel in San Fran. A very interesting week of AGU lies ahead. Sorry you couldn’t make it this year. mike]

Impact of changes in diffuse radiation
on the global land carbon sink

“… estimate that variations in diffuse fraction, associated largely with the ‘global dimming’ period6, 7, 8, enhanced the land carbon sink by approximately one-quarter between 1960 and 1999. However, under a climate mitigation scenario for the twenty-first century in which sulphate aerosols decline before atmospheric CO2 is stabilized, this ‘diffuse-radiation’ fertilization effect declines rapidly to near zero by the end of the twenty-first century.”

December 1st, 2012 at 8:52 PM

Hank, you are one sharp “bird dog!” Thanks–and thanks to Bob, too, for a very interesting comment.

December 2nd, 2012 at 9:14 AM

I havent’ found anything (in my copious five minutes of searching) about whether increased diffuse skylight affects plankton photosynthesis — or has effects on other biology that produces something paleo people look at for markers. Short answer is it seems not too surprising that there’s no reason to expect a clear unidirectional change in tree rings around the time of big volcanic excursions.

Hm, I wonder if there’s any site with trees growing entirely shaded from direct sunlight (thinking of the north side of the Olympic Mountains in Washington).

December 2nd, 2012 at 9:27 AM

and I’m going to shut up and hope some of the scientists come on in.

T Marvell:

December 2nd, 2012 at 4:32 PM

In post 17 I suggested that the fact that volcanic activity does not show strongly in tree rings is due to the fact that volcanic activity mainly reduces temparature in the Summer, whereas trees grow mostly in the Spring. If this is true, one would expect a stronger relationship in the northern lattitudes, where trees tend to grown later in the year. That appears to be the case.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692171/pdf/43XA8LK6PCMVMH9H_353_65.pdf

This could also be due to a “global dimming” effect, which is stronger in higher lattitudes.

David B. Benson:

December 2nd, 2012 at 7:07 PM

Hank Roberts @31 — Better check just how far north the sun is at zenith in the summer (in Washington state).

December 4th, 2012 at 6:09 AM

My impression is that the volcanic coolings – even after major erruptions – are not only underestimated by tree rings but also by ice cores (d18o or melt layers)where the actual volcanic signals (sulfates, ashes etc) were recorded at the beginning. Of course also in ice cores you might then have special problems for the respective years such as particular accumulation or very specific circulation patterns that somehow compensate for the general cooling. But if all different paleo records react just in a way that the climatic effects of the major erruptions are hidden and unobservable then – as you said it – we have a problem.

December 4th, 2012 at 1:40 PM

> sun at zenith
I know, David, that’s the way to locate such possible sources; sun angle shallower than slope so the sun doesn’t reach the site.

I’ve seen steep north slopes where plants get direct sun only a few days a year, if at all. I’ve seen photos of trees growing in deep cave-collapse sinkholes that got no direct sun. Those are rare enough that I doubt anyone’s looked specifically at tree rings from trees that get diffuse skylight but not direct sunlight.

Might be easier to do as a planted field experiment than finding an natural site.

Bob Loblaw:

December 4th, 2012 at 4:21 PM

Hank, David:

Keep in mind that north-facing slopes can see direct sun in early morning or late evening at higher latitudes. Even where I live (at 61 degrees N), on June 21 the sun rises in the NE and sets in the NW, so there are several hours when a N-facing slope sees direct sun. Of course, where I live it’s also pretty flat, so there aren’t many hills to block the sun close to the horizon. Mountain terrain will be different.

recaptcha: ffeywho comments
[Who is ffey, and why is he taking over my comment?]

December 4th, 2012 at 5:47 PM

Bob, yes, I know. It’s rare to find a tree that never gets direct sun; I know they exist, I’ve seen a few that happened to have such bad fortune — I’m sure if they could pull up roots and walk they would!

As you and David note, the shading has to block the sun angle at the longest day of the year.

Are there any faster growing photosynthesizers correlated with tree growth that could be used? If we know how to simulate a volcanic haze event as the plant would see it — Grow one set of plants with no direct sunlight but exposed to enhanced diffuse skylight vs. another set growing in direct sunlight and normal skylight?

flxible:

December 4th, 2012 at 8:13 PM

Hank – Growers of a wide range of plant types [veg and horticulture] commonly use shade cloth on greenhouses, filtering up to 70% of UV – normally to control heat in the house, but the reduction/diffusing of the sunlight doesn’t affect growth.

December 4th, 2012 at 10:29 PM

Flxible, diffused light is described as enhancing growth, for some plants; volcanic events might increase the diffused and lessen the direct sunlight; the two effects might cancel out. Just guessing. Still poking in a desultory way for studies, not finding them.

December 4th, 2012 at 10:32 PM

Ah: Volcano With a Green Thumb
“… environmental biophysicist Lianhong Gu of Oak Ridge National Laboratory in Tennessee and colleagues suspected the decline in carbon dioxide was due to more efficient photosynthesis. Crop scientists have long known that plants use diffuse radiation–familiar to photographers as ‘soft lighting’ –more efficiently than direct sunlight. That’s because diffuse light creates fewer shadows, so it reaches more leaf area, which more than compensates for its reduced intensity.

To test their hypothesis, the researchers studied the exchange of carbon dioxide between the atmosphere and Harvard Forest in eastern Massachusetts. Using a tower perched above the forest canopy, they took measurements of gas exchange in 1992, the year in which Mount Pinatubo had its biggest impact, and compared it with measurements from 1995 to 1997, when the dust from Pinatubo had settled ….”

December 4th, 2012 at 10:35 PM

— More from that abstract:—-

“… photosynthesis on cloudless days was 23% higher in 1992 than in the later years.

The findings should prompt climate scientists to take a much closer look at the impact of diffuse radiation, something they’ve tended to neglect in the past, Gu says. They may also help researchers understand the effects of cloud cover, which also creates diffuse light and represents the biggest source of uncertainty in climate models, he says.

Environmental biophysicist John Norman of the University of Wisconsin, Madison, says the authors make a solid argument for an increase in photosynthesis. And they were lucky to have started taking the necessary measurements just when Pinatubo erupted, he says. “They got the classic case here to make this point.”
— end—-

So — same as the original post, people started off thinking about cooling in the dusty-atmosphere year, and instead found it was plants sucking down _more_ CO2 that year.

I’d guess that means even plankton proxies would have the same issue as tree rings — photosynthesizers for primary production are mostly toward the poles, maybe less influenced by such dust. Guessing again.

Carrick:

December 5th, 2012 at 1:10 AM

Steve Bloom: “This appears to be the link for the lecture, and it’s Pacific Standard time. Anyone who can’t watch it live can see it after 48 hours. I notice there are a large number of other lectures that will be similarly available (link), which is great (and new for this year IIRC).”

Something Steve and I can finally agree on. That’s just too cool to pass up. ;-)

The on-site registration is $495 for this conference, which may seem a bit high, but not really… it’s an incredibly rich and diverse conference. Imagine spending $495 at the world’s top university in geophysical sciences, then being able to pick which courses you want to sit in on today. I’ve always maintained that one gets the best information about the current state of affairs on any topic at conferences, this year’s AGU has been no exception. Kudos to the people who put this “city sized” conference together.

The quality and quantity of light has an impact on photosynthesis but at issue here is can the quality or quantity be so affected by an eruption to make such a remarkable response in tree growth? The answer to these questions depends on how large is the eruption, and how light-limited are the trees? In this regard both of Tim Osborn’s comments above cannot be dismissed. I cannot comment on light dimming or diffusion due to eruptions but I can make a point with regard to the implied affects light quality changes may have on tree growth. The stand structure and composition of a boreal forest in Siberia is different than that of a stand in Petersham MA. (Harvard Forest). This has implications on how far into the canopy light will travel and which plants will benefit from the implied addition. One could imagine a scenario where competitors benefit more, thereby posing a negative growth affect on other plants. It is not a simple dynamic to model and really requires site-specific measurements in the field.

RE: “I don’t have any dog in this particular issue” – Gavin Schmidt.
Gavin – maybe you do not have a dog in this issue, but you certainly threw one a bone with this comment: “Admittedly, not a huge problem and not one that most people, or even most climatologists, are particularly fascinated by but one which threads together many topics (climate models, tree rings, paleo-climate) which have been highlighted here in the past.”

RE: Temperature thresholds. Mann12 require a consecutive 26-day growth spin-up within a 50-60 day growing season, all above 10 degrees c, in order for a tree to grow a ring. If this were the case then how do we explain the stand of trees growing under the Wendelstein Observatory were, since 1960, the average temperature in June, July and August has been 7.98, 10.08, and 10.29 degrees c., the monthly median temperatures are 8.06, 10.06 and 10.15 respectively (BEST, 2012). I choose this station in Bavaria as an example because it has one of the longest uninterrupted records from a high elevation (1833.72 m) where, if you search the web for a picture of the observatory, you can see trees growing next to it. Under the Mann12 hypothesis those trees are living at or below the threshold of existence. I don’t think it is a stretch to say, by the same reasoning, they should not be there.

I know Mike you are receiving a lot of underserved pressure and threats and I sympathize completely. Personal attacks, born out of ignorance, are uncivilized but your peers are neither ignorant nor malicious, we just want to see good science published. One cannot come out and say computer models conclude all high latitude and high elevation tree-ring chronologies have chronic dating errors, without evidence, and think there will not be repercussions.

So I return to the question of our article’s significance. Each of us, the 23 authors of the comment, have different opinions on what that significance is and I speak only for myself. The principles of ecological amplitude and limiting factors, two important components of Evolutionary Theory tell me there must be trees somewhere, at high latitude or high elevation, that grew a ring in those years with volcanic eruptions. The principle of cross-dating, from which Dendrochronology is derived, provides a means by which even a partial ring, in one sample, is enough to correct an entire chronology. I cannot dismiss these principles, which have withstood over a century of study and description, without empirical evidence.

There are currently a number of papers in the publication pipeline spawned by the Mann12 thesis that will pick up this thread. Considering the articles and AGU posters mentioned above, I don’t think the topic will go away time too soon. It is clear there is more to explore and learn on both sides of the debate and that is encouraging.

Herbert Kohl:

December 11th, 2012 at 5:57 AM

AGU PP21B-1977: Widespread locally-absent rings in tree-ring records across the Northern Hemisphere during the last millennium

[Response: Yes, it is well known that local cross-dating rarely yields apparent missing rings. But when entire regions are subject to conditions that cause a missing ring (i.e. because that entire region was below the summer temperature threshold for growth) then local cross-dating cannot identify the missing ring! We’ve made this point repeatedly, but for some reason it doesn’t appear to be sinking in with certain folks. –mike]

December 11th, 2012 at 11:33 AM

> there must be trees somewhere, at high latitude
> or high elevation, that grew a ring in those
> years with volcanic eruptions.

>> local cross-dating cannot identify the missing ring

Is that the definition of “local” for this?

How will dating outside a local boundary work? I gather the idea is to use trees from distant locations to identify which ring is missing — generally the rings match, but in the area affected by the volcano, one does not match?

[Response: Basically yes. The argument is that there is enough climate variability at regional scales that even in a very cold post-volcano year, not all regions will be substantially cooler than usual. So certain regions will collectively be missing a ring, others will not. We’ll have more to say on this soon. –mike]

thanks for the poster link. It answers the question I hadn’t asked yet about the ‘year without a summer’ events.

Is “locally-rings” in the caption text for that a typo?

“Maps showing the distribution and intensity of locally-rings across the Northern Hemisphere ….”

December 25th, 2012 at 3:09 PM

> certain regions will collectively be missing
> a ring, others will not.

I recall Gavin commenting from/after a China trip a while back that most paleo drill core analysis work was local, with a need for correlation work to be done in some consolidated data collection. That would provide other proxies for temperature. Do the tree ring data sets begin to be correlated with other data sets?

Is there a database somewhere trying for a comprehensive list of the various paleo proxies — old and established, and newly being investigated — what time and geographic range they may report, and how they’re being validated and cross-correlated? (sediments, organic residues, isotope ratios; rock and mud and ice drilling work; what else, where else?)